Extendable hydraulic cylinders are commonly used in a variety of mechanisms in a wide variety of equipment applications. These cylinders typically comprise a cylindrical barrel with a piston sealed inside the barrel. A shaft is attached to the piston and sealed to the barrel and typically extends from one end of the barrel, although hydraulic cylinders are also configured with a shaft extending from both ends of the barrel. A port is provided in each end of the barrel, and pressurized hydraulic fluid is directed through the ports to move the piston and extend or retract the shaft.
In a double acting hydraulic cylinder, the barrel is full of oil on each side of the piston, and hydraulic fluid conduits are connected to both ports. Valves connect a pressurized hydraulic fluid source to the double acting cylinder such that hydraulic fluid can be directed into the first port to move the piston toward the second port, and hydraulic fluid on the second side of the piston is pushed out through the second port, and vice versa. The shaft extends or retracts with the movement of the piston. Such double acting cylinders exert force when extending or retracting.
Single acting hydraulic cylinders are used where force is required to be exerted in one direction only. For example hydraulic cylinders are often used on mechanisms used to raise or lower an object. A single acting cylinder can be used to exert a force in one direction only, either extending or retracting, to raise the object by operating a valve to direct pressurized hydraulic fluid through a pressurized port of the cylinder. Hydraulic fluid enters the barrel on the raising side of the piston and pushes the piston in a raising direction. When it is desired to lower the object, the valve is operated to release the hydraulic fluid from the barrel on the raising side of the piston, and the weight of the object forces the hydraulic fluid out through the pressurized port and the object moves down until the valve is operated to stop the fluid flow out of the pressurized port.
Similarly, in trip mechanisms on agricultural implements single acting hydraulic cylinders are used to exert a force in one direction only to maintain a ground engaging tool in a working position. The hydraulic cylinder typically forces a pivoting shank against a stop that is configured such that a ground engaging tool attached to the bottom of the shank is in a desired maximum downward and forward working position when the shank bears against the stop. An active hydraulic source directs hydraulic fluid at a constant pressure through the pressurized port such that the piston and shaft exert a substantially constant bias force on the shank.
The bias force exerted is sufficient to maintain the shank against the stop and the ground engaging tool in the working position during normal operations passing through the ground. When the tool encounters an obstruction, such as a rock, and the rearward and upward force on the tool increases to a level greater than that exerted by the hydraulic cylinder, the ground engaging tool moves upward and rearward against the bias force to clear the obstruction. Once clear, the bias force exerted by the hydraulic cylinder is sufficient to move the shank and attached ground engaging tool back down into the working position.
Thus in single acting hydraulic cylinders, only the end of the barrel on the pressurized port side of the piston contains hydraulic fluid. The opposite un-pressurized end of the barrel contains only air. If the un-pressurized end is sealed the air will expand and compress according to the position of the piston, exerting additional forces on the piston and adversely affecting the desired operation of the hydraulic cylinder. A breather is therefore provided that allows air to enter or leave the un-pressurized end of the barrel as the piston moves back and forth in the barrel. Such breathers typically are screwed into a port on the un-pressurized end of the barrel, and provide a screen through which air passes into and out of the barrel.
Problems arise with conventional breathers in that fine screens can often plug up such that air will not pass through them as required for proper operation, while coarser screens can allow dust to enter the barrel and damage the walls and seals of the hydraulic cylinder. As well moisture can condense in, or be drawn into, the barrel, especially when operating in humid conditions or rain, causing rust and corrosion and premature failure of the cylinder.